THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

[18:00:23]

WOLF BLITZER, CNN ANCHOR: Happening now: Is coal country digging Bernie Sanders? West Virginians gave Hillary Clinton a big win eight years ago. Eight years later, he is hoping to make it almost heaven and grab yet another victory against the front-runner.

ANDERSON COOPER, CNN ANCHOR: COHEN: Warren gets her war on, the outspoken Democratic senator taking on Donald Trump on his own turf, Twitter.

BLITZER: Breaking news, more exit polling results coming in from tonight's two primaries, what voters are saying about the choices they're making and what that says about the general election.

COOPER: There is also breaking news on Donald Trump's search for a running mate.

We want to welcome our viewers in the United States and around the world. I'm Anderson Cooper.

BLITZER: I'm Wolf Blitzer and you're in THE SITUATION ROOM.

We begin this hour with voting under way in Nebraska and West Virginia, with Donald Trump facing his first primary night unopposed, with Ted Cruz talking about possibly getting back into the race, and over on the Democratic side, the front-runner, Hillary Clinton, facing the possibility of another defeat a week after losing Indiana.

That said, she just got a big boost from a high officeholder. Here's what the vice president, Joe Biden, told "Good Morning America"'s Robin Roberts.

(BEGIN VIDEO CLIP)

JOSEPH BIDEN, VICE PRESIDENT OF THE UNITED STATES: I feel confident that Hillary will be the nominee and I feel confident she will be the next president.

(END VIDEO CLIP)

BLITZER: And by the delegate count, that's a likely outcome.

She's campaigning tonight in Louisville. These are live pictures coming in from the event. You can check it out. She is expected to speak any moment now. We will take you live once she begins. Kentucky, by the way, votes a week from today.

So does Oregon, where Bernie Sanders is holding a late-night event tonight in the city of Salem.

Brianna Keilar is there for us right now.

Brianna, even if Sanders wins today, he is unlikely to dramatically cut into Hillary Clinton's delegate lead. Isn't that correct?

We're looking at 29 delegates up for grabs here -- or -- sorry -- in West Virginia, as Bernie Sanders comes here to Oregon tonight to speak to a crowd, I should say, of hundreds. I'm looking at them wrapped around this armory that we're at here in Salem.

And those delegates are going to be awarded proportionally. So, Hillary Clinton will end up picking up some delegates. And that means that for Bernie Sanders it's going to be difficult to close the margin, which at this point is considerable. It's about 300 delegates, a little more than that, that he trails behind Hillary Clinton in just the pledged delegates.

Now, at the same time, because of that, you're seeing some people look at the Sanders campaign and feel that what he's doing instead is fighting for the message, fighting to influence the Democratic platform. You can -- I think, Wolf, if you listen to him -- and we heard him last night in Sacramento -- you don't necessarily always pick that up, because we heard him taking on Hillary Clinton on Wall Street again, drawing attention to her paid speeches from Wall Street, hundreds of thousands of dollars that she brought in after she was secretary of state.

And also we have heard him touting his poll results just recently. The Quinnipiac University poll in some key states, Pennsylvania, Florida, Ohio are showing Bernie Sanders performing better against Donald Trump than Hillary Clinton, and this is something that he's been touting, Wolf.

BLITZER: Certainly has been.

Brianna, is the Clinton campaign at all worried about having to fight on two fronts against both Bernie Sanders and Donald Trump, for that matter?

KEILAR: Certainly, some of her allies are worried about that, Wolf, and what they'd like to see instead is Bernie Sanders helping Hillary Clinton pile on Donald Trump.

Bernie Sanders isn't quite there yet. You certainly hear him taking on Donald Trump a lot and we have heard a number of his speeches where he's taking on Donald Trump a lot more than he's taking on Hillary Clinton, but he's still fighting through this primary and still fighting for the message. Of course, the Clinton campaign I think would like for this to move right on into the general election, but also with a number -- or with the Sanders campaign being seen as having such an uphill climb, you also see some of his attacks on Hillary Clinton getting less attention.

And that has also played to her benefit.

BLITZER: All right, Brianna, thanks very much.

And now to the breaking news on Donald Trump's running mate's search.

CNN's Jim Acosta has been working his sources. He's joining us now.

What can you tell us, Jim?

JIM ACOSTA, CNN SENIOR WHITE HOUSE CORRESPONDENT: That's right, Wolf.

We can confirm that the Trump campaign has told us through multiple sources, to my colleagues who also cover the Trump campaign, that Donald Trump is now down to five -- to six names on his vice presidential search list.

[18:05:03]

At this point, they're not close to naming a pick, I'm told by sources, but at the same time, what's been told to us is that Chris Christie, the New Jersey governor, who was once Donald Trump's rival, and then became his first major endorser, he is still on that list, Wolf.

So, New Jersey Governor Chris Christie is still being considered. Some names we can take off the list, obviously, Marco Rubio. The Florida senator, he took his name out of the running and said yesterday in a statement he's not interested in that job. And Mike Pence, the Indiana governor, put out a statement. I was talking to his office earlier today.

They say he is not interested in that job. But keep in mind, Wolf, they may be early on in this process. Corey Lewandowski, the campaign manager, it was just announced today that he is going to be leading the vice presidential search for Donald Trump. So, that just goes to show you they're at the early stage in this process, Wolf.

BLITZER: They certainly are. It's a big, big job.

All right, Jim Acosta, thanks very much -- Anderson, over to you.

COOPER: Wolf, thanks very much.

Back with the panel, David Gergen, Nia-Malika Henderson, Gloria Borger, John King, also our political commentators left and right, Paul Begala, Van Jones, Jeffrey Lord, and Ana Navarro.

Gloria, what do you make of his reporting?

GLORIA BORGER, CNN SENIOR POLITICAL ANALYST: Well, look, I think it's early and it's very odd that a presidential campaign and a candidate actually says, you know, we have kind of narrowed it down to five or six.

Usually, the candidate doesn't talk about it very much in current history. So, this is different.

COOPER: Usually and this candidate...

(CROSSTALK)

(LAUGHTER)

BORGER: Yes. Exactly. That's what I was going to say.

You read my mind. So, also, Corey Lewandowski is running the process, which is also unusual.

JEFFREY LORD, CNN POLITICAL COMMENTATOR: I guess this means Michelle Fields will not make the list.

(LAUGHTER)

BORGER: But, I mean, you're a -- this is -- he's a political guy. He's a campaign manager.

COOPER: David, does it surprise you?

(CROSSTALK)

BORGER: Usually, a lawyer does it.

(CROSSTALK)

BORGER: David is speechless.

DAVID GERGEN, CNN SENIOR POLITICAL ANALYST: I just -- I don't know, fellow.

I see the pictures. I got the shoving incident and all the rest of that, but usually you have a heavyweight who knows other heavyweights and can do the peer review that needs to be done before you chose somebody.

LORD: That's right.

GERGEN: And it's fine to have Corey...

(CROSSTALK)

VAN JONES, CNN POLITICAL COMMENTATOR: In some ways, it's weird, because your first big choice is V.P., but the choice of who does the V.P. also sends a signal.

(CROSSTALK)

GERGEN: I agree.

(CROSSTALK) JONES: It's sending a weird signal.

(CROSSTALK)

JOHN KING, CNN CHIEF NATIONAL CORRESPONDENT: You have the most unorthodox candidate of our lifetime has a very unorthodox process to pick a running mate...

BORGER: Right.

KING: ... but is publicly talking about sort of very run-of-the-mill, predictable, almost, I need a Washington insider.

COOPER: Although Corey Lewandowski could take a page out of Dick Cheney's book.

BORGER: John McCain had a great vetting process, if you will all recall. He had fabulous Washington, D.C., lawyers do the vetting process.

And then, suddenly, he couldn't get the -- he couldn't choose the person he wanted, which was Joe Lieberman, because his people said you can't do that, you can't pick somebody who is not of our party. So, what did he do? He went and he picked somebody who hadn't really been vetted. And that was Sarah Palin.

(CROSSTALK)

GERGEN: I want to go back to Van's point, because it's really, really important.

This is the moment when you first show people what kind of president you would be, and what kind of people would be around you.

BORGER: Yes.

(CROSSTALK)

GERGEN: Nobody knows who -- if Donald Trump goes to the White House the big question is, who is going with him?

(CROSSTALK)

KING: David is exactly right. And I mean this with no disrespect. Dana is exactly right, according to all of those rule books we have had to shred, then burn and then bury. Trump has redefined everything we're doing.

(CROSSTALK) COOPER: It also seems, Jeffrey, that -- again, I don't know how Donald Trump runs his business, but it does seem he has a very small core group of people.

We all think of the Trump Organization as this massive thing.

LORD: Right. Yes.

COOPER: It seems to me, the little I have seen of it, there's a core group.

We just had Michael Cohen on. Core group of people who are around Donald Trump who are fiercely loyal to him. It seems like Corey Lewandowski is now in that group.

LORD: Right.

COOPER: And so it doesn't really surprise me that he would go to that group to vet.

LORD: No. No.

And I would just say, with all respect to David, that when you look back in history, people whose names are now legendary and iconic and all this kind of thing, once upon a time, when they burst on the scene, with no disrespect to my friend Paul here, but who in 1992 was saying let's leave the big decisions to Begala and Stephanopoulos, et cetera, et cetera? Who would leave the decisions to Bobby Kennedy in 1959?

Who would leave them to Ed Meese in the Reagan era?

COOPER: Well, who did Bill Clinton's vetting process?

LORD: Well, Warren Christopher, who was a heavyweight.

(CROSSTALK)

PAUL BEGALA, CNN POLITICAL ANALYST: Exactly.

But let me make the case for Corey Lewandowski. It's a point that Jeffrey and you were making, which is loyalty. We laughed about Cheney. Cheney was a heavyweight, but he was not loyal to Governor Bush, I believe.

If you read Bart Gellman's book "Angler," which is a terrific biography of the Cheney vice president, the argument is, he manipulated the system. All the leading candidates somehow blew up with information that no else had except the vetters.

[18:10:02]

And, somehow, Cheney was the last man standing. And he himself did not go through the vet. It is extraordinary how he manipulated that.

BORGER: So, Corey is going to be vice president? BEGALA: Nobody believes Lewandowski will manipulate this for his own ends.

He may advise Trump wisely or unwisely, but he will not -- I don't know this -- he will not manipulate with some agenda.

(CROSSTALK)

NAVARRO: He is extremely unorthodox. He's not your typical politician. It's very hard to interpret him. It's very hard to read him.

Corey Lewandowski, for all his shortcomings and all his attributes, has proven to be so far a Trump whisperer, somebody that has got Trump's ear, that knows what Trump wants, that can interpret Trumpism, Trump-speak.

And I think he's not going to make a decision. Trump is going to make the decision.

(CROSSTALK)

NAVARRO: From the menu in front of him.

BORGER: Trump is not looking for a Hail Mary here either.

Trump has said publicly, I'm going to go for somebody who is well- known, who is practiced in the ways of Washington, et cetera, et cetera. So he's narrowed it before they have even started it.

GERGEN: I agree with that.

Listen, I cannot emphasize enough that at the end of the day you don't want somebody who is just simply loyal to Trump. You want somebody who has judgment about human beings and their gravitas and their capacity to be lead.

LORD: And that person would be Donald Trump, though.

GERGEN: And you have to put somebody in the room who is an equal to the people they're talking to. They expect -- these senators expect...

NAVARRO: David, why? He hasn't done it for 11 months and he's the nominee.

(CROSSTALK)

NAVARRO: We're talking in an ideal world, and we're living in a surreal world.

(CROSSTALK)

GERGEN: Well, I agree with that. But do you want Corey running the Defense Department or the State Department?

(CROSSTALK)

NAVARRO: I don't want Corey Lewandowski anywhere near me.

That being said, he can understand and he can read Donald Trump

(CROSSTALK)

JONES: There are some things that don't change.

The job doesn't change. The Constitution doesn't change. The threats facing the country don't change. And just because we have got somebody who has figured out how to run by tweet doesn't mean that -- fundamentally, at the end of the day, these questions are consequential to the country. And they should be handled by somebody who is consequential.

BEGALA: And the voters he needs, not the voters he has. The voters he has love all that "thumb in the eyes, blow it up" stuff, right? They love that.

The voters he needs care most about temperament, judgment, character, places where he's lacking right now. And so that's why perhaps Gergen...

(CROSSTALK)

LORD: But the proof will be in the pudding of who is selected.

BEGALA: Right.

LORD: And I have not heard any out-there names.

BORGER: Right.

LORD: To borrow from President Obama's White House Correspondents -- we're not talking about Miss Sweden here. We're talking about Governor Christie, Governor Kasich, Marco Rubio, whether they want to do it or not. I'm just saying, these are the kind of names that are being floated.

What?

(CROSSTALK)

NAVARRO: You guys have made some beautiful, eloquent cases for what -- how things should be, what should be.

But let's just remember that Corey Lewandowski, his campaign manager, was basically, what, charged in Florida in the midst of the campaign. Donald Trump never backed away for one minute, never so much as apologized

LORD: Loyalty.

NAVARRO: There was never an apology to the woman involved, Michelle Fields, who you just mentioned. So, any other politician should have, would have distanced themselves immediately from somebody that was facing a charge of that nature. Donald Trump does things differently.

(CROSSTALK)

COOPER: He not only does things differently, but actually turns it into a benefit, an asset that people respond to.

LORD: That's right. That's right.

BORGER: And I think, look, Corey Lewandowski is somebody who has been loyal to him.

LORD: He's a smart guy.

BORGER: Donald Trump -- the thing about Donald Trump that we have learned about throughout this entire campaign is that he tells you what he's going to do and he does it.

So, he has said, I'm going to pick a pretty conventional person because that's where I'm kind of short. I don't -- this Washington stuff, haven't been in Washington, never been elected to office.

So the names that have been floated -- and I don't know how accurate they are or not -- all are pretty conventional. Corey Lewandowski is not picking these names. It's Donald Trump who is picking these names. Now, Jones Day is a law firm they have been affiliated with, very big, prestigious law firm which I'm sure will be doing the vetting on a lot of these candidates. But it's not as if...

(CROSSTALK)

COOPER: You could make the issue Corey Lewandowski has been the campaign manager for this entire campaign. This campaign beat all those other campaigns.

(CROSSTALK)

GERGEN: We're moving from a period of politicking to a period of governing. That's what this decision is all about.

COOPER: Theoretically, yes.

GERGEN: Well, but you want the person who would be the best qualified to be the next president of the United States should something happen to you.

And that's what we expect of people. And he can do this any way he wants, but if he wants to gain respect and sort of be treated more seriously as a candidate, as someone who would be a good president, then I think you have to pay some attention...

(CROSSTALK)

COOPER: But can't you make the counterargument, which is he's -- you can argue actually that he's being more responsible than John McCain. For those who didn't like Sarah Palin, that was one of the knocks against her, was that she...

(CROSSTALK)

[18:15:07]

COOPER: ... will be vice president.

GERGEN: But it was a knock against McCain at the end of the day.

(CROSSTALK)

COOPER: Well, it was.

But what Trump is saying is, I actually want the opposite of that. I want somebody who has Washington experience.

BORGER: Exactly. Exactly.

COOPER: Whether it's Corey Lewandowski leading the search or not.

KING: From Trump's perspective, remember, he brought in Paul Manafort. Putting Corey Lewandowski in charge of the V.P. is a pushback at Paul Manafort.

A lot of Paul Manafort's friends are saying, whoa, what happened? You brought in this big heavyweight. He goes back to Jim Baker. He goes back to Gerald Ford. He knows all those people. He knows those heavyweights. He's Trump's relative peer when it comes to age. He's a grownup in the Republican Party. Why isn't it him?

This is what a lot of people in Washington are talking about. If you are Trump, if it ain't broke, don't fix it. He got this far with the way he -- this is the process he wants, and it's his choice.

(CROSSTALK)

COOPER: There's a lot more to talk about, including Marco Rubio, who gave an interview to our Jake Tapper just a short time ago, Donald Trump, what Senator Rubio said to "THE LEAD"'s Jake Tapper late today in his first national interview since leaving the race.

Also tonight, a top Sanders supporter on the senator's chances, in spite of a big shortage of delegates, as well as the question that sounds odd, but a lot of people have been asking it. Would Sanders supporters pick Trump over Clinton if that's their choice in November?

(COMMERCIAL BREAK)

[18:20:33]

BLITZER: Live pictures coming in from Louisville, Kentucky, Hillary Clinton addressing a rally campaign there. Kentucky holds its Democratic presidential primary a week from today. We will monitor what Hillary Clinton is saying, get back them. But, sometimes, I want to bring in our CNN political director, David Chalian. He is with breaking news, more insight into what the voters tonight are really thinking about not only what's happening today, but what's happening in the general election.

DAVID CHALIAN, CNN POLITICAL DIRECTOR: That's right.

In West Virginia we were look looking on the Democratic side here. Issue number one is the economy. And there's high anxiety about the economy in West Virginia. Take a look at this. So, this is the issue with trade.

Do you think that the U.S. doing trade with other countries creates U.S. jobs? Thirty-five percent of those voting in the West Virginia Democratic primary say, yes, it create U.S. jobs; 53 percent says that trade takes away U.S. jobs. That is the second highest that we have seen all season.

Michigan is higher, but that's right up there tied with Ohio. Big, big issue there, trade. And then a look about how worried people are about the economy. Are you worried about the U.S. economy?; 67 percent very worried, 24 percent somewhat worried.

Wolf, that 67 percent very worried, that's a high watermark of the entire season. This state has the highest economic anxiety that we have seen so far in the Democratic primary.

BLITZER: It would help explain why Donald Trump is doing so well. He talks about trade all the time. He talks about the worrisome U.S. economy, he says.

CHALIAN: And these were Democrats. And Bernie Sanders is speaking very much to that message as well.

And so even though it's such a conservative electorate there among West Virginia Democrats, he may find that his economic message especially hitting on trade has some audience there.

BLITZER: All right, David, thanks very much -- Anderson, back to you.

COOPER: Yes. We will see how those exit polls are echoed in the results tonight in West Virginia.

Just before the breaking news hit that Donald Trump has narrowed his list of potential running mates to five or six, former rival Marco Rubio gave his first national interview since leaving the race. Now, we do not know who is on Trump's list, only that a campaign adviser says that no one has been ruled out.

What we do know is that Senator Rubio says he does not want the job.

(BEGIN VIDEO CLIP)

JAKE TAPPER, CNN ANCHOR: Are you saying you would never serve as his vice president if he asked? SEN. MARCO RUBIO (R), FLORIDA: Who is that?

TAPPER: Donald Trump. I'm sorry, Donald Trump.

RUBIO: Because we had to stop.

TAPPER: Right. Right. Sorry.

RUBIO: So, no. He needs to appoint someone to be his vice presidential nominee that more fully embraces the things he stands for.

TAPPER: You mean, no, you're not interested?

RUBIO: Absolutely.

(CROSSTALK)

RUBIO: And, again, to be frank, I have never had those conversations with anyone in his campaign. So I'm not saying that anyone has offered it to me or it's been suggested to me.

I'm just saying to you that I believe he would be best served by someone who more fully embraces the things he stands for, and that's certainly not me.

(END VIDEO CLIP)

COOPER: We're back with the panel.

Ana, it was a fascinating interview that Jake did take, because Rubio is saying he will -- I want to get the language right -- that he would support the nominee, but he did not say outright he would endorse Donald Trump or even that -- Jake pressed him on it -- that he would vote for Donald Trump.

Do you think that will evolve as the election draws nearer?

NAVARRO: If he keeps getting asked about it, at some point, folks are going to have to go from treating Donald Trump like Lord Voldemort, he whose name shall not be mentioned, to the actual Republican nominee and deal with the fact that his name is Donald J. Trump and he's the presumptive nominee and that's what we got.

We don't have any other choices. So, at that point, you have got three choices. You either vote for Trump, you vote for Hillary or you don't vote. That's it. You can parse it as much as you want. You can try to take your time to reconcile your brain with it. You can go to therapy, maybe a little counseling, figure out how to say it in Spanish first, then in English, but those are your choices.

I think Marco -- what you saw in Marco was a guy who was speaking his heart. I think he is wrestling with the idea of Donald Trump with whom he doesn't agree on temperament issues, on character issues, on values issues, on policy issues, with whom he has just competed for the last 10 months, who beat him. All of those things are very hard to deal with for any given person. I think you're seeing Marco wrestle with that and come to terms with it. And I also think it's hard to give Donald Trump a full endorsement, a full-throated endorsement, when what you see from him the first week after he's the nominee is a conversation about Bill Clinton's past infidelities, instead of turning more presidential, like so many of us have wanted him to.

LORD: The thing is, the higher ranking you are in this party, if you have been on that stage with Donald Trump as a candidate, if you're the speaker of the House, the Senate majority leader, you are looked to.

[18:25:07]

I mean, this is your responsibility. The party has selected a nominee. It's your job now to get out there, just as past folks who didn't like, say, George McGovern, when he was the Democratic nominee, or Goldwater. You have got to stand up for the guy.

And, politically speaking -- we were talking about this earlier -- in 1964, when like Nelson Rockefeller and George Romney abandoned Barry Goldwater, that was more or less the end of them politically speaking. Richard Nixon, who stood up there and was for Goldwater all the way when the moment came, went all around the country for him, wound up as president. You just can't run away from this...

(CROSSTALK)

NAVARRO: What is the nominee's job?

(CROSSTALK)

LORD: That's right. Sure. No question the -- and I think Donald Trump will reach out and is talking to people, but it's got to be a two-way street. And these other folks have got to understand, they too are leaders of the Republican Party.

(CROSSTALK)

NAVARRO: But what we heard from him this week is, we don't need a unified Republican Party.

OK, Mr. Trump, I'm with you.

BEGALA: I expected Senator Rubio to begin the interview with Jake by saying, well, I talked to Donald Trump the other day, he called me.

And he didn't say that. I don't know whether Mr. Trump has called, but he should have. Ronald Reagan reached out first to Gerald Ford, only man to ever beat him in an election. Then he put George Bush on the ticket. Barack Obama put Joe Biden on the ticket, one of his rivals. John Kerry put John Edwards..

(CROSSTALK)

COOPER: Is that really Donald Trump's style?

(CROSSTALK)

BEGALA: No, but if you want party unity, it needs to be.

COOPER: Paul Ryan came out in that interview again with Jake Tapper last week, said what he said about not being ready to support Donald Trump.

BEGALA: Right.

COOPER: There were those who said, well, Donald Trump should have then called him and maybe started a conversation. Instead, Trump...

(CROSSTALK)

LORD: When George Bush got out of the race for president in 1980, he called Ronald Reagan, or sent him a telegram in those days.

JONES: One of the things -- this may be much more significant than we really realize.

We may be missing the forest through the trees here. Why does Donald Trump need a party at all? This may not just be Donald Trump ran over the Republican Party. He may be running over the idea of parties, period.

Donald Trump has a party in his hand. He can do all of the activities that parties used to do. He can mobilize voters. He can make arguments. He can raise money with this thing. He may have disintermediated the idea of a party. And so, if that's where he is, that then explains some of his behavior.

BORGER: But he does need them. He does need them for money.

(CROSSTALK)

JONES: We will see. We will see.

(CROSSTALK)

COOPER: Organization.

GERGEN: He can't have it both ways. And that is to go to the party and say raise all the money for me, and not work with the party and to reject the party.

(CROSSTALK)

BORGER: He does. He absolutely needs a party to raise the money for him and, by the way, to go out and campaign for him. And he also has to come out and say, how about those down-ticket races?

(CROSSTALK)

JONES: Is that true anymore? Because he just ran over all these guys.

(CROSSTALK)

NIA-MALIKA HENDERSON, CNN SENIOR POLITICAL CORRESPONDENT: But I think, in a general election, you certainly need infrastructure in the states...

(CROSSTALK)

LORD: Who is going to show up at the door?

(CROSSTALK)

COOPER: We're going to have more with our panel ahead.

Also ahead, Senator Elizabeth Warren is the latest target of Donald Trump's Twitter insults, but she's fighting back against attacks and the nickname he's come up with for her -- up for her. How Senator Warren is going toe to toe with Trump in a war of words -- next.

(COMMERCIAL BREAK)

[18:32:51] BLITZER: As we've seen time and again, Donald Trump certainly very fond of lobbing insults on Twitter, but possibly, he may have met his match. Massachusetts Senator Elizabeth Warren is one of the targets of his more recent tweet storms, and she says, her words, that she's handling it the way you have to handle any bully, by not backing down. Senator Warren says Trump has built his campaign on racism, sexism and xenophobia, and she's taking a very strong stand against him.

The Twitter spat dates back to May 3, after Trump won the Indiana primary and became the presumptive nominee. Warren promising to fight her heart out "to make sure Donald Trump's toxic stew of hatred and insecurity never reaches the White House."

One minute later, he slammed Warren again, this time bringing up her heritage, a controversy going back to her 2012 Senate campaign. Trump tweeting, "Let's properly check goofy Elizabeth Warren's records to see if she is Native American. I say she's a fraud." Warren has long defended her Native American heritage, even after her Republican opponent, Scott Brown, accused her of making it up to get ahead in her career. Seconds later, this: "Goofy Elizabeth Warren, Hillary Clinton's flunky, has a career that is totally based on a lie. She's not Native American." A while later, this: "Goofy Elizabeth Warren and her phony Native American heritage are on a Twitter rant. She's too easy. I'm driving her nuts."

(on camera): But was he really driving her nuts? Warren certainly did not back down. In fact, she took to Twitter again, unleashing a firestorm, accusing Trump of lying his way through the primaries without being held accountable, and putting him on notice those days are over.

[18:35:04] (voice-over): First she called his tweets lame. Then this, in response to him calling her "goofy Elizabeth Warren." "'Goofy,' @RealDonaldTrump? For a guy with 'the best words,' that's a pretty lame nickname. Weak!"

Within 13 minutes, Warren fired off 11 tweets aimed at Trump, calling him "a bully who has a single play in his playbook -- offensive lies thrown at anyone who calls him out."

Another saying he "spews insults and lies because he can't have conversation about his dangerous vision for America." Warren continued, "You can beat a bully, not by tucking tail and running, but by holding your ground. If you think recycling Scott Brown's hate- filled attacks on my family is going to shut me up, @RealDonaldTrump, think again, buddy. Weak."

Her rant ended at 8:49 p.m. with this doozy, "Whatever @RealDonaldTrump says, we won't shut up. We won't back down. This election is too important, and he won't step foot in White House."

Randi Kaye, CNN, New York.

(END VIDEOTAPE)

COOPER: ... with our panel. Van, it's interesting that Senator Warren is really one of the few Congress members to kind of go after Donald Trump in this way. Particularly, she's been very quiet, obviously, in the primary battles between Clinton and Sanders.

JONES: I'm sorry. I'm fan drooling a little bit. I'm swooning.

COOPER: Music to your ears?

JONES: I love that woman so much.

UNIDENTIFIED FEMALE: Really?

JONES: She's the -- in a lot of ways everybody loves Bernie, but she's the one we want to go to the prom with if you're on the left. And so we're, "OK, fine, she doesn't want to go to the prom." You know, people on the left, we'll go with Bernie, but she's there.

NAVARRO: That's a good point, because you and Bernie would make a hell of a couple. JONES: My only point is that this is -- there's a fire and an

authenticity from her that people just love on our side.

COOPER: John -- John.

KING: Why now? Is she trying to position herself to be the -- does she know Harry Reid is leaving the Senate and she wants to take a bigger leadership role in the United States Senate?

I always said -- I've said this before when Bernie -- when she said she wasn't going to run, Bernie Sanders jumped in and stole her car and drove away, and it's become the Sanders wing of the party. Does she want her car back? What does she want?

BEGALA: I think it's more about mission than ambition. First off, if you want to move up in the Senate that's an inside game. You don't go to Twitter for that.

She's -- she's really concerned that the Democrats take a populist economic message to this election, and I think what she's doing is showing she's doing that thing. She's saying, "Here's how do you this, Hillary. Here's how you do this, Bernie."

LORD: How much of Hillary Clinton's message was discussed there? In other words, what that reminds me of is what happened...

COOPER: Is this a marker, sort of she's setting down a marker against Donald Trump?

LORD: You know, I suppose you could look at it that way, but to me, it looks like what happened to Ted Cruz. I mean, he got Ted Cruz off tangent here, and suddenly, Ted Cruz goes off on this, and it had no relevance. And it didn't help him. And suddenly, here is Elizabeth Warren engaged in a Twitter war. To what end? I mean, in terms of helping Hillary Clinton.

NAVARRO: The end is called relevance, and the -- you know, the ironic part about it is that she took a page out of Donald Trump's playbook.

BORGER: Exactly.

NAVARRO: How do I do this the most effective and quick way? Through Twitter. And I think, you know, she is trying to go back and be relevant. At some point, she has got to be having the same exact thought as Joe Biden. What if? If you are Elizabeth Warren, and you're seeing how much trouble Bernie Sanders is having -- Hillary Clinton is having shaking off Bernie Sanders, you have to think to yourself, "What if I had run against Hillary?"

LORD: Van would be here with his super PAC.

GERGEN: She does not have to worry about being relevant. She's extremely relevant to the future of the Democratic Party. Everybody knows that Bernie Sanders is now going to gradually leave the stage. She is going to...

BORGER: He's not going anywhere.

GERGEN: For a long, long time I felt, you know, Hillary-Elizabeth Warren ticket, no, that's impossible. After this you have to start wondering. I think Ana's onto something...

HENDERSON: She's more gifted, I think, than Bernie Sanders at delivering this message.

GERGEN: There are two things about a vice president. One is you want to have them -- that vice president take the fight to the other side and she...

JONES: She can do it.

GERGEN: ... gets under Trump's skin. And that's a really interesting asset. The other thing is that it's possible that all the excitement we've been saying has been missing with the Hillary campaign, if you had two women...

BEGALA: That's not allowed in the Constitution.

GERGEN: Yes, I know.

JONES: There's so many good women in the Democratic Party that it's almost embarrassing. It's almost embarrassing. You have Amy Klobuchar. You've got Kristen Gillibrand. You have -- what's so amazing is I've never seen men do this. All women said it's Hillary's turn. Have you ever heard any man say, "It's your turn"?

BORGER: Not Marco Rubio to Jeb Bush.

NAVARRO: Must you bring that up again?

[18:40:07] JONES: But part of what I think could happen here is, listen, there is an excitement level that these young voters would have if an Elizabeth Warren was on the ticket that would put a lot of this "Hillary can't do this; Hillary can't do that." I think Hillary Clinton, if she's elected, will be one of the best presidents ever. As a candidate, she's having a challenge. Elizabeth Warren fixes a lot of that.

NAVARRO: I can think of few things that would unite the Republican Party more than the idea of a Clinton/Warren ticket.

LORD: Right.

GERGEN: That would be a hell of a campaign.

BORGER: I don't think Bernie Sanders is going anywhere. Bernie Sanders, if he's not the nominee, which we don't assume he will be, will go back to the United States Senate. He will have a huge impact on this convention. He will have a lot of leverage with Hillary Clinton. He will be a chairman of a committee if they -- you know, if they -- if they get the Senate. He will be an important person in Democratic politics. And he will be Hillary Clinton's person in the Senate, who will say to her, "We need to do this, because I got millions of votes."

HENDERSON: It also just goes to show how big a team Hillary Clinton is going to have, how big a cheering section, right? There she was on Twitter making a case against Donald Trump in terms of him being sexist and racist. That was probably not a message that Hillary Clinton wants to make. It's probably not even a message that Barack Obama wants to forward either.

But I think she is going to be part of that team. Barack Obama's going to be. Michelle Obama is going to be out there hugely, as well as Bernie Sanders.

BEGALA: But polls still matter.

HENDERSON: Yes.

BEGALA: Paul Ryan can't even bring himself to say he's for Donald Trump. Here, Hillary has not even locked it up yet, formally, and she's got...

(CROSSTALK)

COOPER: We've got to go. Ahead, will the state -- we've got to go. Will the state where coal is king, what are the results going to be there tonight? Will it crown Bernie Sanders? The Democratic race expected to be tight in West Virginia. I'll talk to the Sanders campaign ahead.

(COMMERCIAL BREAK)

[18:46:39] BLITZER: Voters in West Virginia, they're having their say today. Twenty-nine delegates are at stake for the Democrats. Demographically, it's a strong state for Bernie Sanders. That said, the Mountain State voted overwhelmingly for Hillary Clinton back in 2008 primary, crushing then-Senator Barack Obama.

CNN's Jean Casarez is joining us in Charleston, West Virginia, tonight.

Jean, what's the latest over there.

JEAN CASAREZ, CNN CORRESPONDENT: Well, Wolf, they're saying a lot. I want to tell you right now that there's really an uptick at this polling station. So many people are getting off work and they're coming to cast their vote, but it's really been constant all day. The secretary of state for West Virginia says that a record has already been met with early votes cast, that they have had more ballots turned in for early voting than any other election primary.

And what we're hearing from the voters, they're engaged. It's all about the economy, and in the same breath, they're saying it's the coal industry. And what I've really learned today, so many people, not only the miners and their families, but I talked to a voter who is a supplier to the coal industry, he lost his job, talked about machinery, how those people are losing their jobs and the consultants, the clerical staff, the attorneys that negotiate the contracts, the state workers that are involved in the mining industry are all effected and have been. Even a local woman that owns a restaurant told me her catering business is down significantly as well as business dinners because of the industry itself.

I spoke with a voter who is a born and raised West Virginian. He comes from a coal mining family and his mother was actually born in a coal mining camp. He says the state has to realize the past is coal mining, the future is other industry, that miners need to be retrained and there needs to be diversification. He believes Hillary Clinton is the person that can do that.

I spoke with a young Syrian immigrant here that voted today. She believes it's all Bernie Sanders.

BLITZER: All right. Jean Casarez on the ground for us in West Virginia. Thank you.

As we've said, Bernie Sanders could get a lift from independent voters in West Virginia. They're allowed to vote in the primaries. Senator Sanders has done well in other states with open primaries.

Joining us now is Tad Devine, senior media adviser for the Sanders campaign.

Tad, thanks very much for joining us. Here is the math, even if Sanders does win tonight, he trails Hillary Clinton, as you know, by 303 pledged delegates and 777 overall when you combined pledged, superdelegates. CNN estimates show that Sanders would need 101 percent of all remaining delegates to clench the nomination.

But if Bernie Sanders can continue to win and today will be a big test to that, you know, I think the Democratic Party is going to take a step back when the voting is done and they're going to say who is the strongest candidate. I mean, we've seen in the polling that came out today in the context of a general election Bernie Sanders is by far a stronger candidate than Hillary Clinton, and I think the Democratic Party does not want Donald Trump to be our next president and the best way to prevent that is to run our strongest candidate.

So, we'll make the case to superdelegates. We'll make the case to voters once the voting is done, and I think it's a very powerful case.

BLITZER: Have any of those superdelegates contacted your campaign and switched their alliances?

[18:50:04] DEVINE: No, they haven't, Wolf. But, you know, we've always planned to make that case after the voters have spoken. We think we should let the voters speak first. We should hear what they have to say all the way through California and the District of Columbia, and then after they have spoken, we will make the case and I think the case will make on the polling that came out today. We saw our battleground state polls where Hillary Clinton has an unfavorability 20 points higher than her favorability in all three battleground states that came out today.

And I think that's kind of information that will move these people, superdelegates. They are very sophisticated. So, we'll make the case once the voting is done and every voter had a chance to speak.

BLITZER: As I'm sure you heard, the vice president, Joe Biden, a man I'm sure you admire. He said earlier today, he feels confident that Hillary Clinton will be the Democratic nominee and will eventually be the next president of the United States. You want to react to what the vice president said?

DEVINE: Listen, the vice president and president have been great. You know, neither have endorsed a candidate. I think that really has helped us a lot. I think the vice president has been honest about everything he said to the process the way he sees it.

A week or so ago, he said he favors Bernie's approach of aspiration to Hillary's approach of incrementalism. So, I think He's been evenhanded in terms of the compliments that he's given out and I think they both have been fair and that's really the only thing we asked of both the president and vice president.

BLITZER: And very quickly, Trump, as you know, is the presumptive Republican nominee. He's attacking Hillary Clinton.

Do you worry that staying in the race could weaken the Democratic nominee if it's Hillary Clinton in the general election?

DEVINE: No, I don't. I think the exact opposite is true. You know, in California alone, they've had over 850,000 people newly register or change their registration to vote. And I think by the time we're done registering people in California, we'll get over 1 million people into the process as new registrants. Those people are going to vote in the Democratic primary where there will be a lot of voters and that's going to help to elect Democrats and put more Democratic names on the ballot in November.

I think what Bernie is doing is adding to the process, not detracting from it.

BLITZER: Tad Devine, thanks very much.

DEVINE: Thank you, Wolf.

BLITZER: And just ahead, what to look for when the early results start coming in tonight. John King is standing by over at the magic wall.

(COMMERCIAL BREAK)

[18:56:34] ANDERSON COOPER, CNN ANCHOR: In about 30 minutes, polling stations will close in West Virginia. Nebraska voters have about two more hours to get to the polls. The outcome in Nebraska's primary is nonbinding for Democrats. The state has already caucused and chosen Sanders.

Donald Trump now the only candidate left in the Republican race, obviously. As the results in, there will be clues to watch for. John King is at the magic wall to break it down.

John, what are we looking for tonight, and what's the likely impact on the all important delegate map?

JOHN KING, CNN CHIEF NATIONAL CORRESPONDENT: Anderson, no drama left on the Republican side. Both Ted Cruz and John Kasich, the final two candidates announce they suspended their campaign so it's Donald Trump and Donald Trump alone. The next question, I guess, is we'll watch for the results to see if there is buyers' remorse, any evidence of a protest vote inside West Virginia.

But Donald Trump looks for a big win tonight, looks to advance his march toward 1,237, the magic number in Republican delegates.

On the Democratic side, Bernie Sanders expecting a win. The question there is how big, because if it's a close win, it won't do much to erase Hillary Clinton's delegate lead. If it's a big win, Bernie Sanders can at least claim he's closing the gap.

And in that regard, remember the 2008 Democratic primary. Almost eight years ago to the day, look what then-Senator Clinton did against Senator Obama. Of course, he ultimately went on to be the nominee. Most expected at this point he would be the nominee but this was a late season victory, if you will, for Hillary Clinton and allowed her to say, hey, I can stay in this race.

This is what Senator Sanders was looking for tonight -- something like that and something like this to win the entire state to say, sure, my math is hard but I'm going to stay in.

Now, let's get to that. Let's assume Senator Sanders has a big night tonight. The question is, as he makes the argument, I'm going to continue on, can he cut into the math? First on the Republican side, just imagine this -- West Virginia votes tonight, these delicates are technically unbound. But Donald Trump is the presumptive nominee. Donald Trump will win them all if he wins big in West Virginia tonight and starts to move out here.

Nebraska also votes among Republicans, another state where Cruz would have done well if he were still in the race but he's gone. So, assume those delegates go to Donald Trump. Look what happens here. Maybe no drama but moves the map especially when you add in these unbound delegates, 66 of them so far by CNN's count that have said, we're not bound technically but we're going to vote for Donald Trump on the first ballot. Donald Trump could end the night, Anderson, look at this, 1,150 closing in on the magic number of 1,237 in commitments well earlier than we anticipated just days ago, just a week or so ago. So, the march toward 1,237 for Donald Trump will continue tonight.

On the Democratic side, Senator Sanders, of course, says he's into the end. He wants a win tonight for momentum. This is where the Democratic proportional rules come into play because if Senator Sanders wins, look where he is now. He's down 300 pledged delegates. This is just pledged delegates. He's down 300.

Imagine if he won tonight 55-45. Well, if he wins 55-45, that's a ten-point win. That's a big win. He'd only shave three off the delegate lead.

So, Hillary Clinton would suffer an embarrassing loss maybe especially because she won the state eight years ago, but Bernie Sanders wouldn't cut into the math. So, he needs a bigger win. He needs a bigger win to cut a little bit her lead. But if he cut ten, it would be 290 or so.

So, Hillary Clinton making the case, maybe you're going to shave a little off Senator Sanders, but in the long march, I will get there to the end. But for Senator Sanders, he wants a win to keep supporters energized. He wants a big win to shave off at least a little bit of Hillary Clinton's pledged delegate math. That is the biggest drama in the race really.

Donald Trump's march will continue tonight. The only question among the Democrats is, can Bernie Sanders win with a big enough margin to make a statement as the Democratic race moves on? Anderson?

COOPER: John, thanks very much. That does it for us. Thanks for watching.